THERMAL ION MEASUREMENTS ON BOARD INTERBALL AURORAL PROBE BY THE HYPERBOLOID EXPERIMENT

Hyperboloid is a multi-directional mass spectrometer measuring ion dis tribution functions in the auroral and polar magnetosphere of the Eart h in the thermal and suprathermal energy range. The instrument encompa sses two analyzers containing a total of 26 entrance windows, and view ing in two almost mutually perpendicular half-planes. The nominal angu lar resolution is defined by the field of view of individual windows a pproximate to 13 degrees x 12.5 approximate to. Energy analysis is per formed using spherical electrostatic analyzers providing differential measurements between 1 and 80 eV. An ion beam emitter (RON experiment) and/or a potential bias applied to Hyperboloid entrance surface are u sed to counteract adverse effects of spacecraft potential and thus ena ble ion measurements down to very low energies. A magnetic analyzer fo cuses ions on one of four micro-channel plate (MCP) detectors. dependi ng on their mass/charge ratio. Normal modes of operation enable to mea sure H+, He+, O++, and O+ simultaneously. An automatic MCP gain contro l software is used to adapt the instrument to the great flux dynamics encountered between spacecraft perigee (700 km) and apogee (20 000 km) . Distribution functions in the main analyzer half-plane are obtained after a complete scan of windows and energies with temporal resolution between one and a few seconds. Three-dimensional (3D) distributions a re measured in one spacecraft spin period (120 s). The secondary analy zer has a much smaller geometrical factor, but offers partial access t o the 3D dependence of the distributions with a few seconds temporal r esolution. Preliminary results are presented. Simultaneous, local heat ing of both H+ and O+ ions resulting In conical distributions below 80 eV is observed up to 3 Earth's radii altitudes. The thermal ion signa tures associated with large-scale nightside magnetospheric boundaries are investigated and a new ion outflow feature is identified associate d to the polar edge of the auroral oval. Detailed distribution functio ns of injected magnetosheath ions and ouflowing cleft fountain ions ar e measured down to a few eVs in the dayside.